SURFACE-CHEMISTRY OF COLLOIDAL GOLD - DEPOSITION AND REOXIDATION OF PB, CD, AND TL

Lead, cadmium and thallium are deposited on colloidal gold particles in aqueous solution. The deposition occurs through the reduction of the metal ions by excess electrons which are donated to the gold particles by free organic radicals generated radiolytically. The absorption spectra of the composite particles are measured and compared to calculated ones (using Mie theory and the optical constants of the pure metals). - Au-Pb particles are stable and well separated, and there is reasonable agreement between experimental and calculated spectra. Reoxidation of the lead mantle by Ag+ ions leads to a silver mantle, although about 5% of the deposited lead is not oxidized; this is attributed to the strong binding of Pb adatoms to the gold surface. The reoxidation of Pb adatoms by Ag+ occurs much more slowly than the oxidation of individual Pb(n) particles, which is attributed to the more electropositive nature of the deposited metal. - Au-Cd particles tend to agglomerate, especially when the Cd mantle is reoxidized by Ag+ ions. The resulting Au-Ag particles contain a large percentage of non-oxidized cadmium; alloy formation seems to be a possible explanation. - The spectra of Au-Tl particles are not in good agreement with the calculated ones; it seems that the reduced Tl does not form an even mantle around the Au(n) particles.